ASCE 7-05 Out-of-plane Seismic forces 1

[SteveGregory]

In ASCE 7-05 section 12.11.1, the out-of-plane forces for structural walls (shear and/or bearing) are 0.4SdsIWp with a minimum of 0.1Wp.

These forces are also used for anchoring or bracing the top and bottom of the the wall sections. A minimum anchorage load is given in section 12.11.2.

If a flexible diphragm is used for a C, D or E category building, the anchorage force doubles to 0.8SdsIWp (section 12.11.2.1). The code does not specifically state that the wall should be designed for out-of-plane forces (bending and shear) to this magnitude, only the anchorage.

Is this how you read it? Do you design the wall reinforcing and thickness for 0.4SdsIWp for all situations unless wind controls?

 

[haynewp]

I read it the same way, the anchorage is designed for the greater force but not the wall. Note also the steel anchorage increase required in 12.11.2.2.2.

"Do you design the wall reinforcing and thickness for 0.4SdsIWp for all situations unless wind controls?"

Yes

 

[SteveGregory]

Thanks Haynewp.

As for out-of-plane wind loads, I use the Components & Cladding wind load. When the wind blows in this direction, this wall can't normally contribute any shear resistance as a part of the MWFRS. To calculate the effective area for C&C, I use the Height H between the support levels to get the area = H*H/3. Is this what everyone else does?

 

[haynewp]

If I remember correctly, the H*H/3 is allowed only when the actual area for C&C is less than this value.

 

[SteveGregory]

I got H*H/3 from the Definitions in 6.2. The other option would be to multiply the height by a strip width of 1 foot, 16" or perhaps the the rebar/grout spacing.

EFFECTIVE WIND AREA, A: The area used to determine GCp. For component and cladding elements, the effective wind area in Figs. 6-11 through 6-17 and Fig. 6-19 is the span length multiplied by an effective width that need not be less than one third the span length.

 

[haynewp]

Right, "need not be less than one third the span length".

Basing it on a 1 ft strip x H would be quite conservative. I could also see an argument made for the width being based on the reinforcing spacing.